In the high-pressure processes for preparing melamine, generally, urea melt and if appropriate gaseous ammonia, in the absence of a catalyst at temperatures, for instance, between 325 and 450° C., preferably between 350 and 425° C. and pressures between 50 and 250 bar, are concentrated into liquid melamine and off-gas principally consisting of ammonia and carbon dioxide, with small amounts of gaseous melamine. This reaction mixture is then worked up, depending on the process in various ways, at high pressure and at high temperature. This is followed by melamine solidification, which can be carried out in two ways: in the dry processes, the melamine melt is solidified in the absence of water, with or without ammonia, with simultaneous expansion, cooled and then isolated. However, the disadvantage of these dry processes is that during the expansion many by-products are formed, for example melam and melem, which interfere in the later melamine processing steps, especially in processing to form melamine resins and their secondary products. The condensation by-product melem is particularly interfering.
In a combined dry/wet process according to U.S. Pat. No. 3,637,686, the melamine melt is first solidified with ammonia in the absence of water and the solid melamine is then worked up in aqueous ammoniacal solution to remove the by-products present in the melamine and formed in part during the solidification. However, in such a recrystallization step, the melamine hydrolysis which takes place in parallel to by-product breakdown can mean a considerable loss of yield.
Another possible method for solidifying melamine from a high-pressure process is described in U.S. Pat. No. 3,132,143 or WO 01/36397. There, the reaction mixture obtained directly from the melamine reactor is expanded into a quencher and, in the presence of aqueous ammonia and carbon dioxide, is transferred to a carbon dioxide- and by-product-containing melamine solution. The disadvantage of the process described is that in the hydrolytic by-product breakdown in the presence of carbon dioxide, owing to the relatively low pH, very high temperatures and long residence times are required in order to achieve the desired degree of breakdown of the by-products.
Another possible method for solidifying the melamine melt from a high-pressure process is described in WO 00/29393. There, the melamine melt, using an aqueous ammoniacal solution, is converted to a melamine suspension and the melamine is isolated therefrom. The disadvantage of this process is that, as in the dry processes, during the solidification step, by-products are additionally formed which can be removed from the melamine suspension only by very long residence times in the aqueous phase, which means high melamine loss due to hydrolysis. Furthermore, in the suspension procedure, the risk of deposits and blockages due to by-products crystallizing out is disadvantageous.